Stringed musical instruments typically generate sounds of specific pitches by the vibration of stretched strings. In one common arrangement, which is used, for example in guitars, several strings, side by side, are stretched from an anchor near a tail of the guitar body, over a bridge that supports the strings, and up along a fretted neck to an attachment at the head with a device for adjusting the tension of each string. By holding a string down onto one of the frets, a player defines the length of string between the bridge and the chosen fret, and thus chooses the note to be played.
In an acoustic guitar, the vibration of the string, assisted by construction of the guitar body, which is at least partially hollow, causes sound in the air. That sound can then be detected by a microphone and amplified. In an electric guitar, the vibration of the string is converted into an electrical signal by a transducer, and the electrical signal is amplified and then converted into sound, usually by a loudspeaker. The commonest form of transducer is an electromagnetic pickup, in which the vibration of the strings, which are typically at least partially metallic, within a magnetic field induces an electric current in a coil of wire. The amount of current generated is correlated to the sound that is produced. Electric and acoustic guitars sound very different. That is important, because a particular sound may better suit a particular style of music, or a particular musician.
A “hybrid” guitar is a guitar in which the vibration of the strings is converted into an electrical signal by a piezoelectric sensor in the bridge. The piezoelectric sensor may be the only way of producing an output, or may be used in addition to an electromagnetic electric-guitar pickup, or may be mounted on an amplified acoustic guitar. Hybrid guitars tend to produce a sound somewhere between an acoustic guitar and a conventional electric guitar.
The configuration of the instrument body also affects the sound. Arched top and flattop guitars are two well-known guitar constructions. As a function of traditional construction techniques known to those who are versed in the art of building both flattop and arched top acoustic guitars, the arched top guitar's construction uses an arched or carved top plate that is typically braced with only two braces, as the arched structure is sufficient to withstand the downward pressure exerted by the strings on the top through the bridge. This construction creates a strong initial “attack,” which is the initial transient response when the string is plucked, that decays rapidly, causing the typical arched top guitar to have a short duration to the length of time that the string sounds, which is commonly called “sustain.” Flattop guitar construction uses a non-arched, flat top plate normally significantly braced to withstand the pull of the strings on the guitar's bridge. This construction typically results in an instrument with a less pronounced initial attack than an arched top guitar, but one with significantly more sustain—the plucked note sustains for a longer duration than with an arched top construction.
Many modern guitarists are drawn to the physical and cosmetic attributes of arched top guitars, but are unable to utilize them for live performance or recording as such guitars lack the desired sustain and full range frequency response associated with flattop guitars. These issues are compounded in existing hybrid guitars. Typically, the bridge in a hybrid guitar has a slot in its top surface, which is a “saddle” on which the strings actually rest. The piezoelectric element is located in the bottom of the slot, under the saddle. However, that location can result in inconsistent contact between the top surface of the slot and the underside of the saddle, and thus inconsistent transmission of the vibration from the strings. There is a desire in guitar aficionados to have an instrument with the physical characteristics of an arch top with the sound and response of a flattop guitar, when played both acoustically and amplified.
One remedy that has been attempted to make an arched top guitar respond more similarly to a flattop guitar has been the use of a bridge top section made entirely of metal. That compares with conventional methods of making the base section that contacts the guitar's top, and which often provides an anchor point for a mechanism to adjust the height of the strings relative to the face of the fingerboard, made of metal. Commonly used metals for entirely metal arched top bridge top sections are zinc, aluminum or brass. Using an all-metal bridge top section does not appreciably alter the response of an arched top guitar so as to be more similar to that of a flattop guitar. The resulting guitar tends to sound less warm tonally (lacking in low frequencies) than a typical flattop acoustic guitar, and the sustain of the plucked note is not appreciably extended towards the longer sustain found in typical flattop acoustic guitars.
Another remedy that is common is to install a long piezoelectric pickup strip below a full-length string saddle in a wooden bridge top section, and alter the frequency response electronically using filters and equalization. However, that is not satisfactory, especially with a two-piece adjustable bridge, where the bridge top section is usually mounted to the base section by adjustable posts at both ends, and unsupported in between.
A typical wooden bridge top section suspended between two posts will flex, especially when slotted to hold an acoustic style saddle. That creates problems in two ways. The flexing bridge top section means that the slot can bend or bow under string pressure such that the piezoelectric element no longer being in equal contact across all strings, resulting in differing amplified output for each string and differences in the tone of each string when amplified. The flexible bridge can also absorb certain frequencies of vibration, lessening sustain.
There is still a need for a hybrid guitar bridge that contributes tonally to the sound of an arched top guitar to give it sound characteristics more similar to an acoustic guitar of flattop construction, and for a hybrid guitar bridge with more consistent string-to-string output when using a piezoelectric pickup element that is sandwiched between the string saddle and the bottom of the saddle slot for amplification purposes.